I used my suspected date of ovulation based on my chart to calculate my duedate. However, due to the length of my cycle, the day was only “off”/later by 2 days, I think. If you have irregular cycles or long cycles it can make such a huge difference. I gave birth at 40 and 5, which is the exact average for first time moms. I loved Evidence Based Birth’s article on due dates.
There is a difference because I chose to have a homebirth and my provider was already very much knowledgeable about due dates, the differences in methodology, etc. An expected due month is so much more realistic. I also knew I was not going to be induced for my date, even if we went significantly “over” (I just would have been doing non-stress tests), so I didn’t have to fret, but I know many moms for whom a difference in calculation has made a big difference and they have had to fight with their providers to not be induced early when they think their babies are much littler than the LMP would suggest or that they are barely term when they are reaching 41 or 42 weeks when their provider is getting nervous and want to induce.
I could be totally wrong since this was not the case in my pregnancy, but if you choose to have an early ultrasound, my understanding is that they will almost always date the pregnancy based on that ultrasound and not based on LMP. However, anything later than 10 weeks will not be accurate to the day and it is an awful idea to change duedate based on ultrasound past that point (there is another EBB article about this).
I’d also say if you wind up with a provider who does things in a completely non-evidence based way and you actually talk about it and they won’t budge or think it is a silly thing to nitpick… they probably aren’t the right provider and probably have other non-evidence based relics of bygone medical training that will come out, too. It can be a good indicator of somebody’s philosophy of care if you bring up, “I’d rather consider this date as my duedate because I used FAM and know with accuracy which date I likely ovulated,” and they look at you like you’re crazy or they scoff or tell you no. Always pay attention to the way your provider makes you feel and go elsewhere if you are getting anything other than compassion from them.
Overall, I would say it is a better litmus test for your chosen provider than it is something that is likely to make a huge difference in your pregnancy unless your cycles are widely variant.